Acetylene dibromide

With bromine, to form acetylene tetrabromide (CHBr2CHBr2) or acetylene dibromide (CHBr=CHBr)... 

See Ethyldibromoarsine. a p-DU>romoethyU>enzene See Styrene dibromide. sym.-Dibromoethylene See Acetylene dibromide. un8ym.-Dibromoethylene (1 -Dibromo-ethylene, vinylidene bromide)... 

Phenylpropiolic acid. This is an example of an aromatic acetylenic acid, and is made by adding bromine to the ethylenio linkage in ethyl cinnamate, and treating the resulting dibromide with alcohobc potassium hydroxide which eliminates two molecules of hydrogen bromide ... 

The conversion of the dehydrotrimer 135 into the corresponding bis-cuprate followed by coupling with dibromide 131 (Cadiot-Chodkiewicz conditions) gave the expanded [5]pericycline 122 in 53% isolated yield (Scheme 28) [4]. The more versatile approach by simple oxidative cyclooligomerization of dehydrooligomers of type 135 under high dilution conditions as shown in Scheme 28 provided the acetylene-expanded [3]- 82, [5]- 122 and [6]pericyclines 163 in reasonable to excellent yields [4,7]. 

There are no liquid alkynes which can be conveniently prepared by the elementary student. Some of the properties of acetylenic hydrocarbons may be studied with the gas, acetylene. Although the latter may be prepared in moderate yield by the addition of ethylene dibromide to a boiling alcoholic solution of potassium hydroxide or of sodium ethoxide,... 

Petrov and coworkers [41] showed that the reaction of dibromides of alkenephos-phonic acids with acetylenic alcohols involved an acetylene-allene rearrangement. The products so formed hydrolyzed easily to the corresponding phosphinic acids. The latter on heterocyclization afforded 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives (Scheme 11). 

By starting with styrene dibromide, the yield of phenyl acetylene is reduced to about 64%. 

Alkynes react with bromine via an electrophilic addition mechanism. A bridged bromonium ion intermediate has been postulated for alkyl-substituted acetylenes, while vinyl cations are suggested for aryl-substituted examples.119 1-Phenylpropyne gives mainly the anti addition product in acetic acid, but some of the syn isomer is formed.120 The proportion of dibromide formed and stereoselectivity are enhanced when lithium bromide is added to the reaction mixture. 

Alkenes, alkynes, and arenes, e.g., ethylene, acetylene and benzene, respectively, react (1) with bromine by addition, e.g., ethylene dibromide C HiBri (1.2). acetylene tetrabromide C2H .Br4 1,1,2,2. hexabromocyclo-hexane QHgBr also carbon monoxide yields carbonyl bromide COBr2i (2) with hypobromous add by addition. e.g., olefins form, for example,... 

There are numerous examples of the addition of HBr to alkynes.10 Addition to acetylene is difficult, tends to produce mixtures of bromide products, and requires a catalyst.94,10 1 22 Simple terminal alkynes react with HBr in the absence of peroxides to produce mixtures of 2-bromo-l -alkenes and geminal dibromides (equation 110).85,159 The latter can be produced in high yield when excess HBr is used. 60... 

The loss of a bromide ion when situated in a -position to a carboxylate group occurs very readily under mildly basic conditions. Thus in an alternative synthesis of phenylacetylene (Expt 5.24), cinnamic acid dibromide is converted into /1-bromostyrene under the influence of hot aqueous sodium carbonate solution. Dehydrobromination to yield the acetylenic compound is then achieved in the usual manner with potassium hydroxide. 

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